This invention generally relates to hole plugs and more particularly relates to a self-locking plug for plugging a hole defined by a surrounding structure, which structure may be a nuclear power reactor pressure vessel core barrel flange.
Although hole plugs are known in the prior art, it has been observed that prior art plugs have certain operational problems associated with them which make these plugs unsuitable for plugging a hole formed through a nuclear reactor pressure vessel core barrel flange. However, before these problems can be appreciated, some background is necessary as to the structure and operation of a typical nuclear power reactor and its associated core barrel flange.
In this regard, a nuclear power reactor is a device for producing heat by controlled fission of fissionable material contained in fuel assemblies. A plurality of the fuel assemblies are grouped in a sealed reactor pressure vessel to define a nuclear reactor core therein. Pressurized liquid moderator coolant is caused to circulate through the pressure vessel and over the fuel assemblies for assisting in the fission process and for removing the heat produced by fission of the fissionable material contained in the fuel assemblies.
Disposed inwardly of the pressure vessel is a core barrel for supporting the fuel assemblies thereon, the pressure vessel and the core barrel defining an annular space therebetween. The core barrel has a core barrel flange for connecting it to the inside of the pressure vessel so that the core barrel is supported by the pressure vessel. The core barrel flange has an access hole therethrough for providing access to a radiation detector disposed in the space between the core barrel and the pressure vessel. The radiation detector is typically a neutron detector or activation foil for measuring the integrated neutron flux seen by the pressure vessel. It is important to measure this integrated neutron flux in order to determine the margin remaining to the maximum allowable integrated neutron flux, also known as the "nil ductility temperature" limit, which has safety significance. Mounted atop the core barrel is an upper support structure for supporting reactor internal components. Although the access hole is open during reactor service operations to retrieve the radiation detector therethrough, the hole is closed during normal reactor operation so that the coolant does not flow through it. It is desirable not to have the coolant flow through the hole during normal reactor operation because coolant that flows through the hole bypasses the core and is therefore unavailable for cooling the core. It is desirable to adequately cool the core to avoid damage to the fuel assemblies.
In order to prevent bypass flow through the hole defined by the core barrel flange, a prior art plug has been inserted into the hole to plug the hole. During normal reactor operation the upper support structure bears against this plug to maintain the plug in the hole. However, in order to retrieve the detector during reactor service operations, the upper support structure is removed to provide access to the core and detector. During detector retrieval operations that necessitate removal of the upper support structure, any coolant pump transients occurring at that time may be forceful enough to generate hydraulic forces that may cause the coolant to forcibly dislodge the prior art plug from the hole before it can be grasped and removed from the hole. That is, when the upper support structure is removed, the plug may become dislodged from the hole due to the transient hydraulic force of the coolant acting on the plug because the prior art plug is not secured or locked to the core barrel flange. Such inadvertent dislodgement of the plug from the hole prior to being grasped and controllably removed is undesirable because such a dislodged plug may become a "loose part" that may migrate in the coolant to damage fuel assemblies and other internal reactor components. Therefore, a problem in the art is to provide a plug for plugging a hole defined by a surrounding structure, such as a reactor pressure vessel core barrel flange, so that the plug is not inadvertently dislodged from the hole by upset hydraulic forces generated in the vessel.
Hole plugs suitable for use in nuclear reactor pressure vessels are known. A plug for altering the flow of coolant in a nuclear reactor is disclosed in commonly owned U.S. Pat. No. 4,591,068 titled "Core Barrel Plug" issued May 27, 1986 in the name of Ralph W. Tolino, et al. This patent discloses a core barrel plug capable of being remotely installed in a port of a core barrel of a pressurized water nuclear reactor for converting the reactor from a by-pass downflow configuration to a by-pass upflow configuration. The plug comprises a body having an expandable cylindrical portion with a movable mandrel disposed in the body. Remote fluid pressurization causes the mandrel to be advanced thus expanding the body into contact with the port. The plug also comprises a locking mechanism to prevent inadvertent release of the plug. Although the Tolino, et al. patent discloses a core barrel plug comprising a locking mechanism to prevent inadvertent release of the plug, this patent does not appear to disclose a self-locking plug for plugging a hole defined by a surrounding structure, as described and claimed hereinbelow.
Therefore, what is needed is a self-locking plug for plugging a hole defined by a surrounding structure, which structure may be a nuclear power reactor pressure vessel core barrel flange.